docs: def format_volume(citation_elements):
      """format volume number (roman numbers to arabic)
  
      When the volume number is expressed in roman numbers (CXXII),
      they are converted to their equivalent in arabic numbers (42)
      """
      re_roman = re.compile(re_roman_numbers + u'$', re.UNICODE)
      for el in citation_elements:
          if el['type'] == 'JOURNAL' and re_roman.match(el['volume']):
              el['volume'] = str(roman2arabic(el['volume'].upper()))
      return citation_elements